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The discovery of the electron: I
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1997 Eur. J. Phys. 18 133
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Eur. J. Phys. 17 (1996) 133138. Printed in the UK 133
The discovery of the electron: I
Nadia RobottiDipartimento di Fisica, Universita` di Genova, via Dodecaneso 33, I-16145 Genova, Italy
Received 2 October 1996
Abstract. This paper describes the process by which the firststudies on discharges in rarefied gases led to the discovery ofthe electron in 1897. Particular emphasis is laid on the debatebetween the so-called aetherial and material theoriesregarding the nature of cathode rays. The paper goes on todemonstrate how the debate was resolved by J J Thomsonwith his proposal of a third hypothesisthe corpuscle (orelectron as it became called). The paper closes with ananalysis of the first measurement of the charge of the electronby J J Thomson in 1899.
Resume. Ce travail etablit comment, a` partir des premie`resetudes sur la decharge dans les gaz rarefies, on put arriver, en1897, a` la decouverte de lelectron. On analyse en particulierle debat qui eut lieu entre lhypothe`se dite etheree etlhypothe`se materialiste quant a` la nature des rayonscathodiques. Ensuite on explique comment ce debat fut clospar J J Thomson, qui proposa une troisie`me hypothe`secelledu corpuscule (ou electron, comme il sera ensuitedenomme). Larticle se termine avec un analyse de lapremie`re mesure de la charge de lelectron realisee parJ J Thomson en 1899.
In 1873 Maxwell made this comment on dischargeprocesses in rarefied gases:
These and many other phenomena: : : are exceed-ingly important, and, when they are better understood,they will probably throw great light on the nature ofelectricity as well as on the nature of gases .
As we will see, history proved Maxwell right. Justa few decades later research of this type led to thediscovery of the electron
1. The first steps
One of the most important challenges faced by physicsin the second half of the 19th century was to understandwhat J J Thomson referred to as the secret ofelectricityor the nature of electricity itself and therelationship between electricity and matter. One pathchosen to do this was to study the behaviour of ararefied gas in the presence of an electrical discharge.The reasons behind the choice were twofold. In thefirst instance this type of study would have led toan analysis of the interactions between electricity andmatter in its evidently simplest state. In the secondplace, for the gaseous state, unlike the other states, therewas a fairly consolidated theory (the kinetic theory)that could be used as a point of reference. In anycase, research of this type could only proceed when(thanks above all to Geissler) vacuum pumps able toproduce pressures of the order of 102103 mm Hgbecame available. At these pressures it was observedthat, while the various areas in the gas behaved indifferent ways depending on the nature of the gas, the
type of electrodes, the shape of the tube, etc, there wasalways a dark space near the cathode, independent ofthe operating conditions. Figure 1 below, taken fromPhilosophical Transactions of 1880 , illustrates anumber of discharge phenomena for pressures around102 mm Hg.
This dark space near the cathode (which had alreadybeen pointed out by Faraday in 1838 ) increased onlyin size as the degree of vacuum increased. At pressuresin the order of 103 mm Hg, the space extended beyondthe anode along the entire length of the tube, becomingthe only phenomenon present.
Once the dark space had been identified as a constantin discharge processes, it became the focus of attentionin an attempt to identify its physical properties. From1860 onwards, thanks above all to the work of Plucker,Goldstein, Varley and Crookes, it was established thatthis dark space was the transit area for somethingderiving from the cathode, which invisible until itmet an obstacle, after which it became evident andcould be perceived. This something, independentlyof the material used for the cathode, had the followingproperties:
(i) it caused phosphorescence in the glass or on anyphosphorescent object placed in its trajectory;
(ii) it was emitted perpendicularly to the cathode andtravelled in a straight line, independently of the positionof the anode;
(iii) it produced chemical reactions, exerted amechanical effect, was deflected by a magnetic field,and created a shadow from any object placed in its path.
So far as concerns the nature of this something,from 1870 two opposing theories were developed.On the one hand, there was the aetherial theory
0143-0807/96/030133+06$19.50 c 1996 IOP Publishing Ltd & The European Physical Society
134 N Robotti
Figure 1. Electric discharge phenomena in different operating conditions.
supported by most physicists of the German schoolstudying the question. Under the terms of this theory,the phenomenon was regarded as an electromagneticprocess, i.e. a wave of small wavelength (this is thereason for the name cathode rays). On the otherhand, there was the material theory supported bymost physicists of the British school, who, using ananalogy with the electrolytic processes studied for sometime, considered the cathode rays to be negativelyionized atoms or molecules (the sign of the chargewas established by the curvature in a magnetic field).Clearly these two theories succeeded in explainingin part the properties of cathode rays discoveredup to that point. It is no coincidence that therewas an immediate conflict between the two theories,above all in experimental terms. Every experimentdesigned and implemented to highlight new propertiesof cathode rays was set up to be able to distinguishbetween these two theories. Despite the objectiveof the various experiments performed, no experiment,taken individually, was sufficient to resolve the debatebetween the two theories, and in fact the debatecontinued at an experimental level until 1897 .
Without entering into details, I shall indicate the mostdifficult phase of the conflict. In 1887 Hertz devised aseries of experiments in order to test the basic hypothesisof the material theory, according to which the cathoderays were electrical in nature . In particular, heverified whether or not they carry a charge and weredeflected by an electric field.
Figure 2 illustrates the apparatus used by Hertz todetect an eventual charge. The vacuum tube was placedinside two coaxial cylinders, and , both connected tothe electrometer. The inner cylinder acted as a chargedetector and the external tube as a screen. Accordingto Hertz, if the cathode rays carried a charge itshould have been communicated by induction across theinner cylinder to the electrometer and detected by it.However, contrary to every prediction of the materialtheory, no charge was signalled.
To verify the eventual action of an electric field,Hertz used an apparatus (similar to the one shown infigure 4) in which the cathode rays were made topass through two metal plates connected to the polesof a battery. If the cathode rays were sensitive to theelectric field, they should have been deflected. Contrary
The discovery of the electron: I 135
Figure 2. Hertzs apparatus to detect an eventualcharge of the cathode rays.
to this prediction of the material theory, there was noperceivable shift of the rays.
Hertzs experiments did not, however, end the debatebetween material and aetherial theories. Quite tothe contrary, the debate not only continued, but nowfocused principally on the problems touched on byHertzs experiments, and eventually led to diametricallyopposed results. Supporters of the material theorywere encouraged by the deflection of the rays in amagnetic field, and at the same time they were able toaccount for Hertzs results in terms of their own theoryby saying that an unexpected effect or an as yet unknownphenomenon had in some way masked the electrostaticcharacteristics of the cathode rays. It is from this pointof view and with the precise purpose of eliminatingspurious effects present in Hertzs experiments, that inmy opinion we must interpret, for example, Perrinsexperiment of 1895 .
Figure 3. Perrins apparatus to detect an eventual charge of the cathode rays.
In 1895 Perrin repeated Hertzs experiment on chargecarrying, but with an important variation. The twocoaxial cylinders ( and ), which in the arrangementadopted by Hertz were placed outside the vacuum tube,in this case were placed inside the tube to eliminate thescreening effect of the glass. The apparatus used byPerrin is illustrated in schematic form in figure 3.
The two metal cylinders ABCD and EFGH had twosmall openings, and , to allow the cathode rays toenter them. The cathode was formed of an electrodeN, and the anode of the protection cylinder EFGH.With this apparatus Perrin observed that when the beamof cathode rays entered cylinder ABCD, invariablythe cylinder became charged with negative electricity.If, however, the equipment was placed in a magneticfield, so that the cathode rays could no longer entercylinder ABCD, the cylinder was not charged. Perrinhence concluded: Cathode rays are therefore chargesof negative electricity.
Despite Perrins results, the material theory stillhad to come to terms with Hertzs objection, when hedemonstrated that the cathode rays were not deflectedby an electric field. Several supporters of the materialtheory had repeated Hertzs experiment but had alwaysobtained the old result. In the meantime, yet anotherobjection to the material theory had arisen.
In 1894 Lenard, one of the most strenuous supportersof the wave nature of cathode rays, used an observationmade by Hertz in 1892 according to whichcathode rays seemed to be capable of passing throughthin metal filmsto design a new type of vacuumtube, in which the wall of the tube opposite the cathode(which in normal conditions blocked the cathode rays)was replaced with a metal film, having small enoughthickness to be passed through by cathode rays (e.g.a thickness of about 0.003 mm for aluminium). Thisisolated the cathode rays from the discharge tube,enabling them to be studied under a wide range ofconditions. Lenard  proceeded with a systematicstudy of the absorption of these rays by the variousmaterials. As cathode rays could pass through metalfilms that were impenetrable to atoms (they were oftenused to separate hydrogen or other gases, on one side,from a good vacuum, on the other), according to Lenardthey could not be considered atoms but had to beregarded rather as waves. This was the state of thedebate on cathode rays when Roentgen announced hisdiscovery of x-rays in 1895 .
136 N Robotti
Figure 4. Thomsons apparatus to detect the deflection of the cathode rays by an electric field.
2. The corpuscle
At the beginning of 1896, when Roentgens firstpapers began to circulate among English physicists, J JThomson (then director of the Cavendish Laboratory inCambridge) was studying the conduction processes ofelectricity in gases. The first thing he did on obtaininga copy of the x-ray apparatus devised by Roentgenwas to verify the effect of these rays on a gas. Herealized that the x-rays ionized the gas, turning it intoa good conductor of electricity. At this point the waywas open to the discovery of the electron. Thomsonchecked whether cathode rays had the same propertiesas x-rays. If this were the case, it would have beenpossible to consider the lack of deflection of the raysin the presence of an electric fieldobserved duringexperimentsas due to the ionization of the gas, whichin some way masked the electric field present. In otherwords, the deflection of the rays was zero becausethe electric field strength was reduced to zero. Bymeans of a series of experiments aimed at establishingthe eventual link between the conductivity conferred onthe gas by the cathode rays and the gas pressure,Thomson arrived at the conclusion that there was aconductivity of the gas, which disappeared very rapidlyas the exhaustion was increased . At this pointThomson repeated Hertzs experiment of 1887, butat reduced pressure (see figure 4), and he observedthe deflection of the rays, even when the potentialdifference (created between D and E) was as small as2 V. Thomson commented : It was only whenthe vacuum was a good one that the deflection tookplace. After verifying that the cathode rays werecarrying a charge and were deflected by a magnetic fieldas well as by an electric field, Thomson concluded asthe only possibility that they are charges of negativeelectricity carried by particles of matter. Then he askedthe new question: What are these particles? Are theyatoms, or molecules, or matter in an still finer stateof subdivision? To answer this question, Thomsondetermined the mass/charge ratio of these particles byusing two different experimental methods.
The first method exploited the deflection of raysboth in an electric and in a magnetic field. The apparatusused is shown in figure 4, where a magnetic field B(of fixed strength) was applied perpendicularly to the
path of the rays, while a variable electric field E wascreated between the plates D and E. Thomson varied Eto that value E0 at which the cathode rays returnedto the undeflected position. Under these conditions thevelocity v of the rays took on the value
v D E0=B: (1)Then the electric field was removed and the radius ofcurvature R of the rays observed and calculated. Onapplying equation (1), Thomson found an expression form=e that contained only observable quantities, namely
m=e D RB2=E0: (2)In the second method, Thomson exploited only the
deflection in a magnetic field. The velocity v of therays was determined, assuming that all the kineticenergy can be transformed into heat, from making useof the formula:
m=e D RB2Q=2W (3)where Q represented the charge passing through asection of the beam in the time unit and W was thekinetic energy associated with it. These quantities weremeasured by Thomson using three different tubes, allof the type devised by Perrin (see figure 3). Thequantity W was measured using a thermocouple ofa known thermal capacity placed behind the centralopening.
These two methods enabled Thomson to obtain avalue for the ratio m=e of the order of 107 g/emu.This value proved out to be independent of the materialused for the cathode, the gas employed, and the pressureapplied. He emphasized, in particular, that it was verysmall compared with the value 104, which was thesmallest value known so far for the mass/charge ratio ofan ion, the hydrogen ion. Per se, this value was rela-tively insignificant unless it was separated into mass andcharge, separately. Thomson, however, suc...